When logging online became possible in China in 1995, the authorities cannot have imagined that a decade later millions of people would crash an internet provider in their efforts to access a website where they could listen to a 27-year-old female blogger having sex.

But that is what happened when the publicity-hungry Muzi Mei released a 25-minute recording of an encounter with her latest lover. The former sex columnist, who shot to fame in 2003 after she started publishing graphic accounts of her many one-night stands on her blog, symbolises the sexual revolution in China. Political freedom may be unattainable, but the bedroom is the one place the government cannot monitor and young people are taking advantage. Not only are they having more sex than their parents ever did, they are doing it far earlier.

A survey by Li Yinhe, China's only female sexologist, shows that 70 per cent of Beijingers have had pre-marital sex, compared with 15.5 per cent in 1989. In the major cities, the average age at which people in the 14-to-20 age group first have sex is 17, as opposed to 24 for those aged between 31 and 40.

The new permissiveness means that being faithful to one's partner is no longer obligatory; a March 2005 survey revealed that a third of young people in urban areas believe extra-marital affairs should be tolerated.

Professor Li, who teaches at the Chinese Academy of Social Sciences, has spent 10 years researching the sex lives of the Chinese, and she believes China will "catch up" with the West in terms of sexual practices within 20 years.

But judging by the 50,000 people who flocked to last month's Sex and Culture festival in Guangzhou city in southern Guangdong Province to browse the latest in sex toys - 70 per cent of the world's total are made in the province - it may be sooner than that.

It is a far cry from the days of the Cultural Revolution, when sex was branded as "decadent". Then, women were banned from wearing skirts and dresses, and the authorities were far more concerned about controlling what people got up to in their spare time.

"There used to be a whole layer of government involved in snooping into people's lives," Professor Li said. "People were fired for having affairs and punished for living with their boyfriends or girlfriends."

Some sociologists believe the policy introduced in 1979 restricting urban couples to having just one child was the spark for the sexual permissiveness. Professor Pan Suiming, of the Renmin University of China, said: "The one-child policy shattered the Confucian belief that reproduction is the only purpose of sex."

But the internet has really fuelled the sexual revolution in China. With more than 100 million internet users and sex education in its infancy, young people turn to the internet for everything from information about sex to pornography, which is illegal in China. In the absence of a pub culture, they also use it to meet partners. Some surveys claim 30 per cent of all one-night stands in China are arranged on the web.

Unsurprisingly, this new-found sexual freedom has a negative side. The number of young single women having abortions has soared: 65 per cent of women terminating pregnancies in 2004 were single, compared to 25 per cent in 1999. Rates of HIV infections are growing quickest amongst the 15-to-24 age group, and the number of couples getting divorced in 2004 was 1.6 million, a 21 per cent rise on 2003. But now the genie is out of the bottle, it seems there is no turning back.